High sensitivity optical measurement techniques are required in the fabrication of electronic and optoelectronic devices. The manufacture of semiconductor devices typically begins with a large substrate wafer of semiconductor material, and a large number of semiconductor devices are formed in each wafer. Because the device manufacturing steps are time consuming and expensive, it is important to the manufacturing process of such semiconductor devices that the physical characteristics of semiconductor device structures only vary within a small process window. In order to attain the earliest possible feedback during production, it is necessary to non-destructively characterize the physical properties of device structures before the device is complete. Importantly, the physical phenomena which governs electronic and optoelectronic device operation often occurs at interfaces. For example, the transistor structure generally comprises a “channel” region near a semiconductor-insulator interface, wherein the electrical properties of the channel are controlled by an externally applied voltage. In production, it is also necessary that the measurement be entirely non-destructive, a requirement which strongly favors the use of optical techniques. The optical signatures of interfacial electric fields occur in the vicinity of strong interband transition features such as the band gap, and typically have small amplitudes, in some cases as small as one part in 106. Typical values of the photo-reflectance signal from semiconductor electronic interfaces range in amplitude from ˜10−2−10−5 (Pollack, 1994; Shay 1970). Unfortunately, widely available optical techniques such as ellipsometry or reflectance do not have the sensitivity required to observe signatures of electronic interfaces. However, these signatures necessarily occur in the optical wavelength range, since semiconductor interband transitions occur in the ˜1-5 eV range. Thus, high sensitivity optical techniques are needed in the extraction of semiconductor interfacial electronic signatures.
The requirement for high sensitivity may be met by a proven class of optical techniques known as modulation spectroscopy techniques. Modulation spectroscopy techniques such as “electro-reflectance” and “photo-reflectance” have exhibited sensitivity to differential changes in reflectivity as small as 10−7. Of the modulation spectroscopy techniques, photo-reflectance is best suited for use in the fabrication of electronic and optoelectronic devices, as it is nondestructive and only requires the sample have a reflecting surface (Aspnes, 1980). The conventional photo-reflectance configuration employs a diode laser pump beam to induce small periodic changes in electron-hole populations. Amplitude modulation of the pump beam is conventionally accomplished with an optical chopper, or by fixturing a polarizer at the output of a phase modulator. A second optical beam, coincident with the modulated pump beam is then used to monitor small sample reflectivity changes using phase locked detection. Thus, the conventional photo-reflectance configuration is a realization of electro-modulation, wherein the electric field is induced by the space charge separation field of the electrons and holes (Pollack, 1996).
A primary problem with conventional photo-reflectometers is that they do not use a probe beam containing wavelengths suitable for characterization of electric fields at semiconductor interfaces. Importantly, modulation of internal electric fields produces sharp derivative-like spectral features near strong interband transitions, known as “critical points,” in the semiconductor band structure (Pollack, 1994). Thus, by using a probe beam with wavelength nearby to at least one critical point in the band structure of the semiconductor structure, photo-reflectance information may be used to characterize the signatures of electronic interfaces.
Another problem with conventional photo-reflectometers is that they do not employ a polarization modulation technique to induce changes in the optical response of the electronic interface. Generally, the optical response of a semiconductor electronic interface is anisotropic with respect to polarization vector. In particular, the optical response depends on the amplitude and direction of the induced electric field (Keldysh, 1970). For example, light polarized in the plane of an electronic interface can accelerate free carriers, producing a sharp third-derivative photoreflectance lineshape, whereas light polarized normal to the plane of a quantum cannot accelerate carriers and typically results in a first-derivative lineshape (Aspnes, 1980). Thus, by using polarization state modulation of the pump beam, and introducing the laser beam at a non-zero angle with respect to the surface normal, an anisotropic optical response may be induced at the electronic interface.
Another problem with common photo-reflectometers is they lack a spectroscopic probe beam (Rosencwaig, 1985, Borden, 2000). Since the photoreflectance signal is obtained at only a single wavelength, the spectral position of critical points, such as those associated with internal electric fields and/or semiconductor band structures, cannot be determined. Thus, while commercial photo-reflectometers may be useful in correlation to electronic carrier densities or thermal effects, these cannot usefully determine internal electric fields or semiconductor band structures.
Thus, while conventional photo-reflectometers and optical spectrometers may be suitable for the particular purpose to which they address, they are not well suited for the characterization of the optical response of semiconductor electronic interfaces. In particular, they do not use a probe beam containing wavelengths suitable for characterization of electric fields at semiconductor interfaces. Moreover, although conventional photo-reflectometers may use a laser pump beam with sufficient power and focusing to create electric field modulation inside the sample, this pump is not polarization modulated so as to induce anisotropic optical responses associated with semiconductor electronic interfaces. The polarization modulation feature of the technique also allows ellipsometric characterization of the semiconductor film structure. Additionally, although conventional reflectance difference spectroscopy has been accomplished using polarization modulation, it has been implemented at normal incidence, providing minimal sensitivity to electronic interfaces. Also, conventional reflectance difference spectroscopy uses lower laser powers.
In these respects, the polarization modulation photo-reflectometer according to the present disclosure substantially departs from the conventional concepts and designs of the prior art, and in so doing, provides an apparatus primarily developed for the characterization of optical signatures of electronic interfaces, including photo-reflectance features at the band edge or other direct or indirect critical points in the band structure.